There has been an immense drive in modern optical inspection systems towards miniaturization. This has been necessitated by the need to access hostile, space-limited or difficult environments. Typical fields in which extreme miniaturization of optical inspections systems are developed are: medical endoscopes, borescopes for the inspection of tubes with small diameters, industrial machinery, a wide variety of systems such as security systems based on optical readout. Also, along with the development and widely available microsystems, in situ inspection methods of objects having sub millimeter size for inspection in difficult accessible confined places are becoming mandatory.
Strategies for miniaturization of optical inspection systems to date have included the use of special fibers or fiber bundles, combined with ingenious micro-fabricated lenses, mirrors and beam splitters.
Endoscopes are an example of such devices, and may be flexible or rigid in nature. Flexible endoscopes rely on image transfer through optical fiber bundles and are particularly important in observations through natural openings into organs. These instruments, also known as fiberscopes, typically focus light through a miniaturized objective or a gradient-index (GRIN) lens that create an aiming on an input facet of a coherent fiber bundle, which subsequently acts as an image guide. In order to feed light into an endoscope a relatively complex optical assembly is needed comprising a beam splitter through which the illuminating light can be transmitted to the light guide while at the same time transmitting the received light beam coming from the illuminated object at the distal end of the endoscope. Endoscopes may also be used for the inspection of parts located in narrow openings of industrial machines. A similar concept called borescope is used for the inspection of pipes. Typical endoscopes, fiber bundle inspection systems or borescopes have a typical cross-section of 5-15 mm. The use of lenses, mirrors or beam-splitters is a fundamental limitation on the size reduction of this kind of optical inspection systems. The miniaturization of concepts using these components is a huge technical challenge, especially when side views of the object need to be obtained.
The document US2013/0317295 explains a light assembly for remote visual inspection that allows to illuminate and visualize an object to the side of the distal end of the light assembly. The device disclosed in US2013/0317295 is limited in that it would be extremely difficult to miniaturize the system so that the overall cross section of the device is smaller than 3-5 mm. In other systems a light conducting fiber is attached to the endoscope, such as disclosed in US2004/00442. The system uses special designed conventional lenses to transmit the image of an object to a CCD camera. Again, a basic limitation of the design is the possible miniaturization of the device because of the use of macroscopic lenses.
In some applications optical systems are used that are based on the optical layout of a periscope. Again here most systems rely on the use of lenses and mirrors and the systems are limited to centimeter sized periscopes having centimeter sized cross sections and it is difficult to make periscopes flexible.
Therefore most optical inspection systems based on classical endoscopes, borescopes and periscopes have a limited use where the insertion volumes are extremely small and they are expensive and not realizable in a batch process. With the advent of MEMS technologies new solutions are being proposed for miniature catheters, such as explained the publication of H. Park et al., “Forward imaging OCT endoscopic catheter based on MEMS les scanning”, Optics Letters, Vol. 37, nr.13, 2012. Although the use of MEMS components allows to make innovative devices such as scanning micro endoscopes, the systems are still limited in site as connections have to be made to fiber optics and GRIN or other types of small lenses are required. The systems based on MEMS are still limited by outer diameters in the range of several millimeters. Because of the component and assembly costs such systems still have a relative high cost.
In a number of applications optical inspections systems are required that may fit into extremely narrow apertures or slots, having lateral dimensions in the sub millimeter range.